Further fix thinko in plpgsql memory leak fix.
There's a second call of get_eval_mcontext() that should also be
get_stmt_mcontext(). This is actually dead code, since no
interesting allocations happen before switching back to the
original context, but we should keep it in sync with the other
call to forestall possible future bugs.
Discussion: https://postgr.es/m/
f075f7be-c654-9aa8-3ffc-
e9214622f02a@enterprisedb.com